This report describes studies on the regulation of intestinal calcium uptake by vitamin D. Impaired intestinal calcium absorption is known to occur in elderly population which contributes to the development of osteoporosis and bone fractures. Using rat as a model, we have shown that duodenal calcium uptake decreases in the senescent rat and the serum concentration of 1,25(OH)(2)D(3), the active form of vitamin D, declines concurrently. This year we attempted to correlate the gene expression of the 1,25(OH)2D3 receptor and calbindin, a protein involved in facilitating calcium absorption with the number of receptors and the calbindin content in duodenum of adult and aged rats. We demonstrated a positive correlation which suggests that the deficiency in intestinal calcium absorption observed during senescence can be attributed to the alteration of gene expression of proteins regulated by vitamin D.